Background Cardiorespiratory fitness is rarely measured in population studies. Most studies of fitness do not examine differences by population subgroups or seasonal trends.
Methods We used a validated submaximal exercise test to measure fitness in 5976 women and 5316 men, residing in England. We expressed fitness as maximal oxygen consumption per kilogram total body mass (VO2maxtbm) and fat free mass (VO2maxffm). Descriptive statistics were computed across anthropometric and sociodemographic characteristics, as well as across the year. Progressive multivariable analyses were performed to examine mediation by physical activity energy expenditure (PAEE) and BMI.
Results Mean±SD VO2maxtbm was lower in women (35.4±7.6 ml·min-1·kg-1) than men (42.1±7.4 ml·min-1·kg-1) but VO2maxffm was similar (women: 59.7±11.8 ml·min-1·kg-1; men: 62.5±10.4 ml·min-1·kg-1). Fitness was inversely associated with age but not after adjustment for PAEE. People in more physically demanding jobs were fitter compared to those in sedentary jobs but this association was attenuated in women and reversed in men following adjustment for total PAEE. PAEE and BMI and were positively associated with fitness at all levels of adjustment when fitness was expressed relative to fat-free mass. Fitness during summer was 4% higher than the winter among women, but did not differ by season among men.
Conclusions Fitness was inversely associated with age but less steeply than anticipated, suggesting older generations are comparatively fitter than younger generations. PAEE and BMI were stronger determinants of the variance in fitness than any other characteristic including age. This emphasizes the importance of modifiable physical activity behaviours in public health interventions.